The development of in vitro models that provide useful, cost-effective tools for assessing long-term electrochemical function of chronically implanted electrodes is proposed. The in vitro models will provide an alternative to animal models for developing experimental protocols and assessing chronic electrode stability. They will allow long-term testing of electrochemical function and stability of electrodes under conditions that reflect the in vivo electrochemical environment without the expense and variability of conducting such testing in animals. The program focuses on electrodes for neural recording, functional electrical stimulation (FES), and chemical sensing in the central nervous system (CNS), although applications to electrodes in the periphery and to biostability of passive implants are anticipated. An inanimate model and a living model employing mouse progenitor cells are proposed. Both models are designed to mimic the compositional and diffusion environment of the CNS as it affects electrochemistry at an electrode. The inanimate model was developed in Phase I. The Phase II program will expand the model to include living cells. The validity of the inanimate and living models will be evaluated by comparison of electrode function in the in vitro models with that observed under identical test protocols in the chronic cat cortex.